The invention relates to a filter assembly for membrane filtration of liquids, and more in particular a spiral wound filter assembly. The invention further relates to an apparatus comprising such a filter assembly, and to methods for using this apparatus.
In the last decades, the industrial application of membrane filtration techniques such as microfiltration, ultrafiltration, nanofiltration and reverse osmosis has expanded enormously. Important application areas are for instance drinking water, wastewater, biotechnology and food. Membrane filtration on an industrial scale mostly comprises cross-flow or tangential filtration, where a cross-flow of liquid is applied along the membrane surface in order to reduce membrane fouling. The opposite of cross-flow is dead-end filtration, which is most frequently applied in lab scale applications.
Membrane filters are filters that are able to separate on a molecular scale, with a maximum separation size of tens of micrometers. These membrane filters are placed in membrane modules, also called membrane units, in order to be able to incorporate them in a process system. Membrane filters and their modules are available in various models such as hollow fibres, flat sheets, tubes and spiral wound modules. An industrial membrane filtration apparatus mostly comprises a plurality of membrane modules, arranged in one or more stages.
In particular spiral wound modules have gained much importance in industrial applications. This is supposed to be due to their compact design (high ratio of membrane surface to module volume, low hold-up volume), as combined with a good overall performance. Spiral wound filter assemblies typically comprise a housing, said housing holding one or more spiral wound filter units in a filter space. The housing is equipped with a feed inlet at one end and a retentate outlet at another end. By applying a pressure drop over the feed inlet and retentate outlet, a cross-flow of liquid along the membrane is induced. The housing further comprises a permeate outlet for draining the liquid passed through the filter. A spiral wound filter unit in the housing comprises one or more membrane filter “envelopes” which are connected and sealed with their open side to a perforated permeate collecting tube. The envelopes are wrapped around the collecting tube, forming a cylindrical spiral wound filter unit. Upon filtration, the filtration liquid is fed parallel to the membrane surface and along the membrane envelope. The pressure difference over the membrane filter (=transmembrane pressure (TMP)) induces filtration of liquid through the membrane filter to the permeate flow path. At the permeate side, the permeate flows perpendicular to the feed flow through the spiral permeate flow path in the membrane envelope. After passing the spiral permeate flow path, the permeate is collected in the central permeate collecting tube and drained via the permeate outlet.
Up to now however, the application of spiral wound membranes is more or less limited to application in filtration processes that are operated at a relatively high TMP. This is due to the compact design as well: the cross-flow of liquid over the membrane consequently causes a (relatively high) pressure drop over the cross-flow channel, from the inlet of the module to the outlet of the module. As a result, the average TMP is also high. This is even more the case in industrial systems, where mostly two or more spiral wound modules are placed in series, resulting in an equivalent increase of the TMP as compared to one module.
It is well known that a good balance between (a high) cross-flow rate on the one hand and (a low) TMP on the other hand is of prominent importance for the performance of membrane filtration systems, as related to the occurrence of concentration polarisation phenomena near the membrane surface (“fouling”). For filtration liquids with a high fouling tendency, this balance is very delicate, which often means that the filtration process cannot be successful without specific measures taken in order to arrive at the desired balance.
For hollow fibre, tubule and tubular membrane systems, such measures are described in literature. An option is for instance to induce a recirculation flow along the permeate side of the membrane. When the flow resistance of the recirculation flow path is increased, for instance by filling the flow path with beads, a pressure drop between the inlet and outlet of the permeate flow path is achieved, resulting in a lower TMP (see e.g. U.S. Pat. No. 6,709,598).
For tubular modules of ceramic membranes moreover, it has been proposed to produce membranes with a decrease in thickness or an increase in porosity in the direction of the cross-flow, in order to induce a uniform flux along the flow path.
In general, these measures for a good balance between cross-flow and TMP, as proposed for hollow fibre, tubule and tubular membrane systems, are not suitable for spiral wound membranes. Firstly, because the membrane material normally is of polymeric nature, without the possibility of controlled variations in porosity or thickness. Secondly, because the specific design of spiral wound membranes does not allow for such measures.
The specific design, and more in particular the fact that the permeate side of the membrane is formed as an envelope, with an opening at one side only, clearly distinguishes spiral wound from other modules such as tubular, tubule and hollow fibre modules. It is for instance clear that it is not possible to apply a permeate recirculation flow in spiral wound modules as proposed for other modules.